1. Field of the Invention
This invention relates to a method and an apparatus for detecting a position through the use of GPS (Global Positioning System) satellites.
2. Description of the Related Art
A GPS-based position detecting apparatus includes a GPS receiver for receiving GPS signals from at least three GPS satellites moving around the earth. The GPS receiver derives positing information from the received GPS signals. When information effective for positioning can be derived from the received GPS signals, that is, when the GPS satellites from which the received GPS signals come are worth tracking, the GPS receiver analyzes the received GPS signals in detail and computes the absolute position (the latitude and the longitude) of the apparatus in a plane on the earth from the results of the analyzation.
In the event that the GPS-based position detecting apparatus is in an underground place or shaded from GPS satellites by a building, GPS signals do not reach the GPS receiver in the apparatus. In this case, the GPS receiver neither tracks at least three GPS satellites nor measures the position of the apparatus. Even in such a case, the GPS receiver is continuously supplied with power to prepare for future normal conditions where the GPS receiver can track at least three GPS satellites. Accordingly, power is wastefully consumed by the GPS receiver.
Japanese patent application publication number 10-020014 discloses a GPS-based position detecting apparatus designed for power economy. In the apparatus of Japanese publication 10-020014, power supply to a GPS receiver is periodically turned on. The GPS receiver implements positioning when the power supply thereto is turned on. In the event that the GPS receiver fails to complete positioning within a prescribed time interval, the power supply thereto is forcedly interrupted. During the forced interruption of the power supply, the GPS receiver does not implement positioning even when normal conditions are restored where the GPS receiver can track at least three GPS satellites.